Method of dehydrating with organic solvent and then vegetable tanning untanned skins



United States Patent Ofiice 2,906,591 Patented Sept. 29, 1959 METHOD OFDEHYDRATING WITH ORGANIC SOLVENT AND THEN VEGETABLE TANNING UNTANNEDSKINS Alexis E. Ushakoif, Beverly, Mass., assignor, by mesneassignments, to Nathan W. Levin, New York, N.Y., Alexis E. Ushakotf,care of Seco Venture, Somerville, Mass, and J. B. Hatton, care ofEagle-Ottawa Leather Co., Grand Haven, Mich., as trustees No Drawing.Application June 21, 1954 Serial No. 438,319

8 Claims. (Cl. 8-94.18)

This invention consists in a vegetable tanning process carried outentirely under non-aqueous conditions, and provides a process which maybe carried out far more rapidly than prior art processes as well as onein which superior control over the leather is afforded.

Vegetable tanning as generally practiced consists in immersing the skinsunder carefully controlled conditions in aqueous vegetable tanninsolutions of gradually increasing strength until the desired amount oftannin has been incorporated in the skin. The skin is thereafter driedcarefully with frequent mechanical manipulation to produce the finallytanned leather. The process is timeconsuming, generally requiringseveral weeks for the introduction of the tannin, and several days fordrying.

The advantages of treating skins in systems utilizing inert watermiscible organic solvent instead of Water have long been appreciated. Aprimary advantage is that the tannin may be introduced into the skinrapidly, generally in the time required for the solution to penetratethe skin. It has, however, not been possible prior to this invention tocomplete the tanning operations under non-aqueous conditions. Prior artprocesses of vegetable tanning using inert organic solvent solutions oftannins have all included the steps of wetting the skins in water afterthey have been impregnated with the tannin solution. The reports onthese processes indicate that a final water treatment is necessary tofix properly the tannin in the skin.

Thus, while prior art reports indicate that the time consumingoperations of depositing tannin in the skin may be avoided by treatingthe skin with an organic solvent solution of tannin, the step ofrewetting in water makes it necessary to dry the skin slowly and undercarefully controlled conditions. Furthermore, rewetting of the solventwet skin in water after treatment with the organic tannin solutionrenders solvent recovery difiicult because of the formation of asolvent-water mixture.

A process of vegetable tanning which may be carried out completely undernon-aqueous conditions is disclosed in applicants copending applicationsSerial No. 100,965 filed June 23, 1949, now abandoned, and Serial No.263,- 129, filed December 24, 1951, now Patent No. 2,781,241, of whichthis application is a continuation-in-part. This invention is based inpart on the discovery that the tannin may be deposited in the skin froman organic solvent solution by altering the solubility of the tannin orthe affinity of the skin for the tannin, and that a finally andcompletely tanned skin may be obtained if the skin is finally acidified,which may be performed either before or after the skin is finally dried.

Accordingly, the process of this invention consists in general ofextracting and replacing the water of an ordinary water-wet untannedskin with an inert organic watermiscible solvent, then impregnating theskin with an organic solvent solution of a vegetable tannin and causingthe tannin .to be deposited in the skin. The skin is'then 2 finallytanned by acidifyingand drying, in the manner indicated.

The initial step of extracting and replacing the water in the water-wetuntanned skin to solvent dehydrateit may be performed by repeatedlyimmersing the skin in the inert water-miscible organic solvent untilsubstantially all of the Water is extracted, as may be determined bymeasuring the water content of the water-solvent mixture in equilibriumwith the skin. Alternatively, the solvent may be forced through the skinunder an applied fluid pressure differential, conveniently in the mannerand by the process described and claimed in applicants copendingapplication, Serial No. 233,924, filed June 27, 1951, now Patent No.2,702,229. The skin is thereafter treated under substantiallynon-aqueous conditions until finally tanned, and may be dried of solventprior to further treatment. When dried the solvent dehydrated skinexhibits much of the softness, flexibility and texture of tanned skins,and may accordingly be inspected and graded prior to being tanned. Afurther advantage of drying the solvent dehydrated skin accrues from therapidity with which the tannin solution penetrates a dry skin during thesubsequent step of immersing or otherwise treating the skin with anorganic solvent solution of a vegetable tannin.

The tannin solution may consist of any of numerous well-known vegetabletannins, such as quebracho, cutch, sumac, wattle and hemlock, to namebut a few, dissolved in a suitable inert organic solvent, such asacetone or methanol. Preferably, the solvent is made acidic to increasethe solubility of the tannin and to permit introduction of the tanningsolution into the skin without causing deposition of the tannin on theskin fibers and consequent clogging.

In this connection it has been observed with respect to inert organicsolvent solutions of vegetable tannins, that under acidic conditions thetannin tends to remain in solution, and that the solubility decreases asthe solution is made more basic. It further appears that as the skin ismade more basic its aflinity for the tannin increases. Thus, inintroducing the tannin into the skin and causing it to be deposited itis preferred that the skin and solution be initially quite acidic (e.g.an acidity equivalent to a pH of below about 2.0) to permit complete andthorough penetration of the tannin, and that the tannin be thereafterdeposited in the skin as by introducing a basic material into thesolvent wet skin, as by immersing the skin in an organic solventsolution of a base or by exposing the skin to vapors of a base.

It has been observed, however,'that in the acidity-basicity rangeequivalent to a pH range of about 20-60, as determined by colorindicators (e.g. Bromphenyl blue and Methyl red), tannin from an organicsolvent solution will penetrate the skin and become deposited therein tosome extent, and the skin may accordingly be at least partially tannedby immersing it in an organic solvent solution of a tannin atacidity-basicity conditions equivalent to a pH of about 2.0-6.0 anddrying it without adding additional base to the skin. If the solventdehydrated skin is treated with the tannin solution under conditionsmore basic than this, the tannin will tend to deposit in greater amountsin the skin as the solution penetrates, with the result that the outerregions of the skin will initially pick up tannin to the exclusion ofthe interior, and'complete penetration of the skin with tannin willrequire greater time. But nevertheless, even with such limitations, thetannin may be introduced into the skin under slightly basic conditions.Preferably, however, as indicated above, the tannin is introduced whilethe skin and solution have an acidity: equivalent to a pH of below about2.0 and the tannin is then caused to become deposited by reducing theacidity to correspond to a pH ofabove 6.0, prior to final acidifica-'tion anddrying.

After the tannin has been deposited in the skin, the skin may be driedand further treated as required, or alternatively the skin may betreated as required prior to drying.

With the tannin deposited in the skin, the skin should be acidic tobring it to its finally tanned condition. A subsequent acidificationstep may not be necessary if the skin is already acidic, as pointed outabove. If, however, deposition of the tannin has involved adding a baseto the skin, further treatment to make it acidic is preferred.Accordingly, the skin may be immersed or otherwise treated with anorganic solvent solution of an acid, which may be done withoutredissolving or otherwise adversely affecting the tannin depositedtherein, apparently because the rate of solution of vegetable tannins isslow. Alternatively, the skin may be exposed to acid vapors after thetannin has been deposited in it, and either before or after the skin isdried. When the acidified skin is finally dried, it will be found to befully tanned and equal or superior as leatherto that produced byconventional vege table tanning processes. An effect achieved by thisprocess which is not achieved readily by aqueous vegetable tannage is auniform distribution of tannin throughout the cross-section of the skin.

Suitable solvents for carrying out the process include generally anyinert water-miscible organic solvent, such as acetone, methanol,ethanol, isopropyl alcohol, methyl acetate, dioxane to name but a few ofthe more common ones. It is obviously desirable to carry the processthrough in the presence of a single solvent to prevent intermixing ofsolvents and the resulting necessity of separating the solvents.

In acidifying the skin and the organic solvent solution of the vegetabletannin prior to incorporation of the tannin solution into the skin,hydrochloric acid is preferred although other mineral and organic acidsmay be used. It has been found that in organic systems a skin willadsorb very large amounts of strong mineral acids, such as HCl, much inthe manner of an anion exchange resin, but only moderate amounts of weakorganic acids such as acetic acid. Thus, because hydrochloric acid isvolatile and may escape from the skin when it is dried, it is preferredover other mineral acids, as sulfuric, which would remain in the skinafter drying and would require subsequent treatment for removal, and ispreferred over weak acids because desirably large amounts may beadsorbed by the skin.

In the final acidification the skin need be made only moderately acidic.Thus, if thefinal acidification involves treating the skin with anorganic solvent solution of an acid, a weaker acid, such as acetic acid,is preferred, because smaller amounts are adsorbed by the skin than whena strong mineral acid is used, although the latter may be usedsatisfactorily. If acidification is done by exposing the dried skin toacid vapors, any volatile acid is satisfactory, such as acetic orhydrochloric, the latter being preferred.

Under preferred conditions, the skin during impregnation with the tanninsolution is maintained at an acidity equivalent to a pH of about 2.0 orlower, but is acidified during the final acidification such that whendried the acidity is equivalent to a pH of between about 2.0 and 6.0,preferably about 2.5 to 4.0.

Suitable bases for treating the skin during processing according to thisinvention include ammonia, either in solution or as a gas,trimethylamine, also either in solution or as a gas, as well as suchother well-known basic materials as sodium hydroxide or sodiumcarbonate. In adding a base to the skin, the skin may be immersed in abasic solution or it may be exposed to the vapors of a basic material.

The foregoing description-is presented as a general outline of theinvention, and limited to the essential steps. Other conventionaltanningoperations may be included, if

desired, such as fat-liquoring which is conyenientlyper-q 4 formed inthe manner described in my copending application, Ser. No. 426,151,filed April 28, 1954, after the tannin has been deposited in the skinand preferably before the skin has ever been dried of solvent.

In general, the several steps may be performed by immersing or drummingthe skins in appropriate solutions until the desired condition isattained, or, where there is no danger of the tannin migrating from theskin, by forcing the treating solution through the skin under a fiuidpressure differential, as by the apparatus and process described inapplicants copending application, Serial No. 233,924, filed June 27,1951, now Patent No. 2,702,229. A convenient method of determining whenthe condition desired by any step has been attained in the skin consistsin cutting a sample of the skin to obtain a fresh crosssection, to whichmay be applied color indicators to determine the degree and distributionof acids or bases through the skin. Similarly, by obtaining a freshcrosssection of a skin the distribution of tannin through it may bereadily determined.

In describing this invention, reference is made .to the acidity of theskin in terms of an equivalent pH value, by which is meant the pH asdetermined by applying a color indicator to the skin, and comparing theresultant color with standardized samples of known pH.

The following examples have been selected as illustrative of preferredembodiments of this invention.

Example I A water-wet pickled sheepskin is washed in water to removeexcess salt and then solvent dehydrated by drumming in successive bathsof acetone until the acetone- Water mixture in equilibrium with the skinis no more than 0.792 at 70 F. The skin is then drummed in five timesits Wet weight of acetone to which has been added about one fifth itswet weight of concentrated commercial hydrochloric acid (22 B.).Drumming is continued until the skin is acidified throughout to anacidity equivalent to a pH of less than 2.0. This may be determined bytaking a sample of the skin, rinsing it in acetone to remove excessacid, then cutting it to expose a cross-section and applying a colorindicator, e.g., thymol blue, to the fresh section. Initially the acidwill be adsorbed at the surfaces while the interior remains at a lowacidity and this will be readily apparent by the color of the indicator.When the skin is thoroughly acidified, as evidenced by an acid reactionwith the indicator through the entire cross-section of the skin, theskin is removed from the acidified acetone, rinsed in acetone to removeexcess acid and then dried of acetone.

The dried skin is then immersed in a methanol solution of quebrachoformed by combining Quebracho extract, cold water soluble ..grams 600Methanol cc 2400 Hydrochloric acid, conc. commercial cc 20 then tumblingthe mixture for 24 hours and filtering out undissolved quebracho (about54 grams).

After the skin is thoroughly impregnated with the quebracho solution, itis removed and immersed in about four times its wet weight of methanolto which is added ammonia, either as a gas, as its concentrated aqueoussolution or as a methanol solution, in an amount at least sufiicient toneutralize the acid in the skin. In this example, 5% of the wet weightof the skin of ammonia gas, or 15% of the wet weight of the skin ofconcentrated aqueous ammonia (28%), added to the methanol is sufiicient.The skin is kept in the ammonia bath until it is uniformly basicthroughout as may be determined by removing a sample, rinsing it inmethanol, then cutting a fresh cross-section to which is applied a colorindicator such as Bromthymol blue. Generally one to two hours in theammonia solution is required, and preferably the neutralization iscarried out in two stages, by

changing. to. a fresh ammonia solution about halfway through theneutralization, each solution containing about half the total ammoniarequired.

After the skin is neutralized, and the quebracho has been deposited inthe skin, the skin is acidified, conveniently by drying the skin andthen exposing it in an atmosphere of hydrogen chloride until the skinindicates acidity uniformly throughout which may be determined byapplying a color indicator to a freshly cut crosssection. The timerequired will depend on such factors as the concentration of hydrogenchloride and the porosity and thickness of the skin, but will generallyrequire in an atmosphere in equilibrium with concentrated hydrochloricacid at room temperature, an exposure period of between 30 seconds andfive minutes.

Example II In this procedure, the skin is treated in the mannerdescribed in Example I to and including the neutralization in theammonia solution. Instead of being dried the skin is finally acidifiedby immersing it in five times its wet weight of methanol containingsufficient acid, such as acetic or hydrochloric, to render the skinacidic (equivalent to a pH between about 2.5 and 4.0) throughout.

Preferably acetic acid is used because of the lower adsorptive capacityof the skin for it, 'as compared with hydrochloric acid, with the resultthat the acid penetrates the skin rapidly. When the skin shows anacidity throughout its cross-section equivalent to a pH of 2.5-4.0, asmay be determined by Bromphenol blue or Bromcresol green, it is removedand dried.

If the acidifying acid is a strong mineral acid such as HCl, it will beadsorbed by the skin initially at its outer surfaces and will penetrateinwardly only as the adsorptive capacity of the outer regions issatisfied. Thus, before there is total penetration by the acid, morethan enough to bring about the necessary acidification will have beenadsorbed. Ordinarily the skin will have adsorbed sufficient hydrochloricacid when the acid has penetrated from each side a distance of aboutonethird the thickness of the skin, as may be determined by applying anindicator such as thymol blue to a crosssection of the skin. If the skinis then dried, the acid at the outer regions will thoroughly permeatethe skin and bring about proper acidification throughout such that thedried skin is well tanned.

Example III A pickled sheep skin is drummed repeatedly in methanol untilthe specific gravity of the equilibrium solution is below 0.805 at 70F., and is then drummed in its weight of methanol to which has beenadded 2% of the weight of the skin of 22 B. hydrochloric acid. The skinis then dried and drummed in a quebracho solution formed by combiningQuebracho extract grams 400 Methanol to make up cc 2000 and tumbling themixture for 24 hours then filtering out undissolved quebracho fractions,adding to the filtrate 290 cc. of a solution of ammonia gas in methanolhaving a specific gravity of 0.777 at 70 F., then boiling the ammoniatedmixture under reflux for three hours, and finally cooling it prior tointroduction of the skin.

After the skin is thoroughly penetrated by the quebracho solution,during which time quebracho will be deposited in the skin, the skin isimmersed and tumbled in its own wet weight of carbon tetrachloride towhich has been added 1% of its wet weight of acetic acid, untilthoroughly penetrated, and the skin is then dried.

In this example the tannin solution was made basic by the addition ofammonia, and as a result the tannin deposited on the skin duringimmersion of the skin in the tannin solution and without subsequenttreatment. The finally dried skin is a fully tanned leather of highquality.

Example IV Cc. Ammonium hydroxide (cone. commercial) 50 Acetone 1000until uniformly permeated as determined by'a color indicator such asthymol blue applied to a freshly cut crosssection, and is then rinsed inacetone to remove excess base. .1

A piece of the skin weighing 464 grams is then drummed in a quebrachosolution consisting of:

Ce. Quebracho solution as described in Example I above 928 Ammoniumhydroxide, conc. commercial 3.1

until thoroughly impregnated with the tannin, and then rinsed in acetoneuntil the washings are clear.

A piece of this skin weighing 240 grams is finally tumbled in an acidsolution consisting of:

Acetone 1000 Glacial acetic acid 4.8

until uniformly acid throughout a cross-section, and is finally rinsedof excess acid in acetone and dried.

Example V A pickled sheep skin is washed in water to remove excess salt,and is then solvent dehydrated by tumbling repeatedly in methanol untilthe liquid in equilibrium with the skin has a specific gravity of lessthan 0.805 at 70 F. The skin is then removed and drummed in its weightof a methanol solution of ammonia having a specific gravity of 0.777 at70 F., until thoroughly penetrated as determined by applying a colorindicator to a freshly cut cross-section. The skin is then drummed intwice its weight of methanol to which has been added 0.4% of sodiumhydroxide based on the wet weight of the skin, until equilibrium isreached. After the skin is rinsed in methanol it is drummed in twice itswet weight of a tannin solution consisting of:

until thoroughly penetrated with the tannin. The skin is then acidifiedby drumming in twice its weight of carbon tetrachloride to which isadded 5% of the wet weight of the skin of glacial acetic acid, untiluniformly acidified, and is finally dried.

The leather produced in each of the foregoing examples is in allrespects fully tanned such that it may be wet in water and dried withoutbecoming hard or horny and will have a shrinkage temperature generallywell in excess of F. In addition to producing fine leather, the processhas the advantages of being rapid and lacking the critical control ofconditions characterizing vegetable tanning processes carried out underaqueous conditions. In this connection it has been found that theconcentrations of the various treating solutions are not particularlycritical so long as sufficient treating reagent is present to achievethe desired condition, and in carrying out the various steps it iscontemplated that one skilled in the art will be able to formulatetreating solutions suitable for performing the step on the basis of theteachings of this specification and without resorting to experimentationamounting to invention.

It is also contemplated that modifications to the process described willoccur to those skilled in the art and that such may be made withoutdeparting from the scope of this invention. In particular, otherwell-known vegetable tanning agents may be used with entirelysatisfactory results as maycertain synthetic materials known to behaveas vegetable tanning agents. One such is benzoquinone which has beenused, as a solution in methanol prepared by adding to 2500 cc. of asaturated (at room temperature) solution of benzoquinone in methanol,2500 cc. of methanol and 40 cc. of a 10% solution of sodium hydroxide inwater, as the tanning agent in the procedure of Example V.

Similarly, the tanning agents derived from sulfite waste liquors, e.g.sodium ligno sulphonate, commercially available as Maratan, and othersynthetic tanning materials, such as naphthalen sulfonicacid-formaldehyde condensation product (available as Leukanol ND.) andthe sodium salt thereof (sold as Tamol) are generally suitable in thepractice of this invention. It is also contemplated that chemicallysimilar materials, not soluble in water and hence not presently used astanning agents, but soluble in suitable organic solvents may also beused with very satisfactory results in the practice of this invention.

Having thus disclosed this invention and described in detail preferredembodiments thereof, I claim and desire to secure by Letters Patent:

1. A method of tanning skins under substantially, nonaqueous conditionsconsisting essentially of contacting an untanned water-wet skin with aninert Water-miscible organic solvent until substantially all the waterfrom the skin is extracted and replaced by said solvent, then processingthe skin in the absence of water with the steps of impregnating the skinwith an inert organic solvent solution of a vegetable tanning at anacidity of the skin such that the tannin is introduced into the skin,controlling the acidity of the skin until the tannin impregnatedskin isbrought to an acidity corresponding to a pH of more than 6, the tanninis distributed through the skin and the skin remains essentiallyuntanned, then finally adding an acidic reagent to the skin to bring thefinal acidity of the skin to correspond to a pH of between 2.0 and 6.0and drying the skin to remove organic solvent so that the skin istanned.

2. A method of tanning skins under substantially nonaqueous conditionsconsisting essentially of contacting an untanned water-wet skin with aninert water-miscible organic solvent until substantially all the waterfrom the skin is extracted and replaced'by said solvent, then processingthe skin in the absence of'water-With the steps of impregnating the skinwith an inert organic solvent solution of a vegetable tannin at anacidity of the skin such that the tannin is introduced into the skin,controlling the acidity of the skin until the tannin impregnated skin isbrought to an acidity corresponding to a pH of more than 6, the tanninis distributed through the skin and the skin remains essentiallyuntanned, then finally adding an acidic reagent to the skin to bring thefinal acidity of the skin to correspond to a pH of between 2.5 and 4.0and drying the skin to remove organic solvent so that the skin istanned.

3. A method of tanning; skins under substantially nonaqueous conditionsconsisting essentially of contacting an untanned water-wet skin with aninert water-miscible organic solvent until substantially all the waterfrom the skin is extracted-and replaced by said solvent, thenprocessing-the skin in the absence of water with the steps ofimpregnating the skin with an inert organic solvent solution of avegetable tannin under conditions such that the acidity of the skincorresponds to 'a pHof less than 2.0, then reducing the acidity of theskin until the tannin impregnated skin is'brought to a pH of more than6, the tannin is distributed through the skin and the skin remainsessentially untanned, then finally adding an acidic reagent to the skinto bring the final acidity of the skin to correspond to a pH of between2.0 and 6.0 and drying the skin to remove organic solvent so that theskin is tanned.

4. A method of tanning skins under substantially nonaqueous conditionsconsisting essentially of contacting an untanned water-wet skin with aninert water-miscible organic solvent until substantially all the waterfrom the skin is extracted. and replaced by said solvent, thenprocessing the skin in the absence of water with the steps ofimpregnating the skin with an inert organic solvent solution of avegetable tannin under conditions such that the acidity of the skincorresponds to a pH of less than 2.0, then reducing the acidity of theskin until the tannin impregnated vskinis brought to a pH of more than6, the tannin is distributed through the skin and the skin remainsessentially untanned, then finally adding an acidic reagent to the skinto bring the final acidity of the skin to correspond to a pH of between2.5 and 4.0 and drying the skin to remove organic solvent so that theskin is tanned.

5. The method defined by claim 4 wherein the acid added in thefinalsteps is in solution in an organic solvent.

6. The. method defined by claim 5 wherein the acid added in the finalsteps is acetic acid.

7. The methoddefined by claim 4 wherein the acid added to the skin tobring the final acidity of the skin to correspond to a pH of between 2.5and 4.0 is added by exposing the skin to vapors of a volatile acid.

8. The method defined by claim 7 wherein the skin is dried before acidis added to the skin to bring the final acidity of the skin tocorrespond to a pH of between 2.5 and 4.0 and the skin is thereafterexposed to the vapors of a volatile acid.

References Cited in the file of this patent UNITED STATES PATENTS2,781,241 Ushakofi Feb. 12, 1957 FOREIGN PATENTS 118,155 Australia Feb.16, 1944 OTHER REFERENCES Progress in Leather Science, 1920-1945,London, 1948, pp. 527-529.

1. A METHOD OF TANNING SKINS UNDER SUBSTANTIALLY, NON AQUEOUS CONDITIONSCONSISTING ESSENTIALLY OF CONTACTING AN UNTANNED WATER-WET SKIN WITH ANINERT WATER-MISCIBLE ORGANIC SOLVENT UNTIL SUBSTANTIALLYALL THE WATERFROM THE SKIN IS EXTRACTED AND REPLACED BY SAID SOLVENT, THEN PROCESSINGTHE SKIN IN THE ABSCENCE OF WATER WITH THE STEPS OF IMPREGNATING THESKIN WITH AN INERT ORGANIC SOLVENT SOLUTION OF A VEGETABLE TANNING AT ANACIDITY OF THE SKIN SUCH THAT THE TANNING IS INTRODUCED INTO THE SKIN,CONTROLING THE ASCIDITY OF THE SKIN UNTIL THE TANIN IMPREGNATED SKIN ISBROUGHT TO AN ACIDITY CORRESPONDING TO A PH OF MORE THAN 6, THE TANNINIS DISTRIBUTED THROUGH THE SKIN AND THE SKIN REMAINS ESSENTIALLYUNTANNED, THEN FINALLY ADDING AN ACIDIC REAGENT TO THE SKIN TO BRING THEFINAL ACIDITY OF THE SKIN TO CORRESPOND TO A PH OF BETWEEN 2.0 AND 6.0AND DRYING THE SKIN TO REMOVE ORGANIC SOLVENT SO THAT THE SKIN ISTANNED.